Heretofore, as an optical fiber, an inorganic glass optical fiber capable of excellent optical transmission over a wide wavelength range has been known and practically used mainly in a trunk line system. However, the inorganic glass optical fiber is expensive, has poor workability, and is not so resistant to bending stress. Therefore, a plastic optical fiber which is less expensive and in which making diameter large, the work of end faces and handling are easy has been developed and put to practical use in some fields of lighting, sensors, and wiring for communication between OA or FA equipment.
Generally, a plastic optical fiber (hereinafter referred to as “POF”) is a fiber comprising a core-sheath structure in which as a core, a polymer having a large refractive index and excellent light transmittance such as polymethyl methacrylate, polycarbonate, polystyrene or amorphous polyolefin and a transparent polymer having a smaller refractive index than that of the core polymer as a sheath are used.
As a industrial production process of such a POF, in general, a core polymer and a sheath polymer are arranged concentrically by use of a multi-component fiber spinning nozzle and melt-spun into a fiber, and the fiber is then drawn under heating for improving mechanical strength.
Of cores to be used in the POF, polymethyl methacrylate is used as a core of a high-performance POF on an industrial scale since it is excellent in transparency, mechanical strength and weather resistance.
However, since the glass transition temperature (hereinafter referred to as “Tg”) of polymethyl methacrylate is as high as 100 to 115° C., its application is limited in view of heat resistance.
For this reason, Japanese Patent Application Laid-Open No. 18608-1983, for example, proposes that a protective layer is further provided around a sheath layer to form a structure comprising three or more layers so as to improve heat resistance.
Further, Japanese Patent Application Laid-Open No. 11128-1993 discloses a technique which improves uniformity in the diameter of a POF by suppressing fluctuations in the diameter of the POF when it is subjected to heat drawing or an annealing after the heat drawing.
Further, Japanese Patent Application Laid-Open No. 16905-1992 discloses a method of improving a transmission loss by heating a POF having a polycarbonate as a core at 60 to 100° C. for a long time.
However, the method disclosed in Japanese Patent Application Laid-Open No. 16905-1992 cannot improve the heat resistance of a POF since the annealing temperature is lower than the Tg of the core by 50° C. or more and the annealing time is short. Further, the invention disclosed in Japanese Patent Application Laid-Open No. 18608-1983 has the problem that even if the heat resistance of a material used in the protective layer is improved, a core itself is thermally shrunk when the temperature used gets close to the Tg of the core. Further, Japanese Patent Application Laid-Open No. 11128-1993 is about the internal structure of a heating furnace used for the annealing, and since an appropriate annealing time and tension when a POF is heated at a predetermined annealing temperature are not set, the effect of reducing the thermal shrinkage of a POF is not satisfactory.
In addition, to improve the properties such as heat resistance of a POF, Japanese Patent Application Laid-Open Nos. 131206-1987, 303304-1988, 68503-1990, 201270-1994, 299912-1987 and the like disclose a method in which a POF is subjected to a non-contact annealing in line after the drawing step to maintain orientation of polymer chains in the POF axial direction which has been provided in the drawing step as much as possible so as to suppress the shrinkage of the POF at high temperatures.
However, since this method cannot remove inner strain of a POF sufficiently, it cannot improve the heat resistance of the POF to a satisfactory level. Further, when the temperature of a non-contact heating furnace is raised to remove the inner strain, the orientation of polymer chains cannot be maintained, thereby causing decrease in the mechanical strength of the POF and an increase in the non-uniformity of the diameter of the POF.
When these conventional POFs are used in optical communication or a sensor in a high temperature environment, e.g., in an engine room of an automobile or the like or in the interior of an automobile in mid summer, they are thermally shrunk, thereby causing decrease in optical transmission properties or trouble in wiring in a connector or the like.